1. Field of the Invention
The present invention relates to optical systems of laser scanning devices, such as laser printers, and, more particularly, to the pre-scan optics assemblies in such devices.
2. Description of the Related Art
Optical systems used in laser printers, and other laser scanning devices, may be characterized as having three sub-systems, namely, a pre-scan optical sub-system, a scanning sub-system, and a post-scan sub-system, each with appropriate mounting hardware for properly positioning the components thereof. A known pre-scan optical sub-system includes a laser diode with large beam divergence serving as a light source, a collimator lens, a pre-scan lens and a plane pre-scan mirror. The collimator lens produces a collimated beam from the light emitted by the laser diode. The pre-scan lens focuses the processed beam to a waist, and the plane pre-scan mirror folds the pre-scan optics path, and attenuates laser power.
The scanning sub-system is essentially a motor driven, rotatable, polygonal reflector, having adjacent peripheral mirror surfaces, or facets, that both translate and rotate during operation of the printer. The mirror surfaces reflect the collimated and focused beam from the pre-scan optical sub-system. The translation direction of the reflector determines the scan direction of the beam passing to a scanned object, such as a photosensitive drum in a laser printer.
A known post-scan optical system includes a focusing lens for transforming the light beam reflected from the polygonal reflector of the scanning sub-system into a beam having spot size suitable for the laser printing operation, and a lens known in the art as an f-theta lens. The post-scan optical sub-system may also include one or more folding mirrors to adapt to the geometry of the printer apparatus.
The pre-scan optical sub-system defines the light beam axis between the laser diode source and the rotatable polygonal reflector, and establishes the beam diameters and curvature on that axis. Although the common optical components used in known pre-scan sub-systems are relatively uncomplicated from a design standpoint, the pre-scan optical sub-system utilizes very short focal length optics of high numeric aperture for reasons of size and efficiency of coupling to the laser diode. As a result, pre-scan optics assemblies are highly sensitive to component tolerance and to placement accuracy. Also, the pre-scan optical sub-system is required to produce a beam waist in the cross scan or processing direction, perpendicular to the scan direction, at a precise location relative to the polygonal reflector. This requirement has been satisfied in the prior art by a plano-cylindrical lens oriented with the axis of the cylinder parallel to the scan direction. The use of aperture stops for reducing the width of the beam in the pre-scan optics assembly is known.
The pre-scan optics assembly of the pre-scan optical subsystem can be considered to include the aperture, the lens, the lens carrier and mounting hardware. Proper alignment of these components is critical to the overall performance of the device. To achieve alignment accuracy, it is known to establish tight tolerances for manufacture and assembly of many components, and/or sub-assembling some or all of the components in fixtures to obtain proper alignment before assembling the components into the printing device. These prior approaches are complex, subject to error and are costly to perform, adding significantly to the overall cost of the printing device.
What is needed in the art is a pre-scan optics design that simplifies assembly, reduces the number of parts requiring tight manufacturing or assembly tolerances, and eliminates the need for costly subassemblies.
The present invention provides accurate alignment of the pre-scan optics for a laser scanning unit by directly referencing datum surfaces of the component parts to mating surfaces in the system base housing. Errors in alignment are minimized by maximizing the dimensions of the mating surfaces, and by reducing the number of parts that make-up the pre-scan optics assembly.
The invention comprises, in one form thereof, a pre-scan optics assembly for a laser scanning device, with a housing adapted to be secured to the device, a channel defined in the housing, and first and second spaced reference surfaces in the housing. A lens carrier is securable to the housing, and has a pocket. A pre-scan lens is adapted to be held in the pocket of the lens carrier, and has a datum surface referenced against the first and second reference surfaces in the housing.
The invention comprises, in another form thereof, a method for assembling a pre-scan optics assembly in a laser scanning device. The method comprises steps of providing a housing, spaced reference surfaces in the housing, and a channel in the housing, the channel having spaced tracks; providing a lens and a lens carrier, the lens carrier having a forward abutment surface; restraining the lens in the lens carrier; urging the lens forwardly in the lens carrier, against the abutment surface; securing the lens carrier in the channel; and urging the lens downwardly against the reference surfaces.
The invention comprises, in still another form thereof, a lens carrier for a pre-scan optics assembly having a housing for receiving the carrier in a laser scanning device. The lens carrier has a body defining a pocket for receiving and restraining a lens therein. Portions of the body are received by and referenced to the housing. The body includes a wall defining an optical aperture of the assembly.
The invention comprises, in a further form thereof a pre-scan optics assembly for a laser scanning device having an optical axis, a scan axis and a process axis, the pre-scan optics assembly having a lens with a primary datum surface and a face surface. A housing has a reference surface for restraining the lens against rotation about the optical axis and the scan axis of the device. A lens carrier has an abutment surface for restraining the lens against rotation about the process axis. The lens carrier is secured in the housing for fixing the position of the lens carrier and the lens.
An advantage of the present invention is providing a simplified pre-scan optics assembly for laser scanning devices that is easier to manufacture and less costly to assemble than previous designs are.
Another advantage is providing fewer parts in a pre-scan optics assembly, and reducing the individual tolerance requirements for individual parts, while maintaining a specified overall assembly tolerance.
Yet another advantage is reducing the need for pre-assembly fixturing of components in a pre-scan optics assembly.
Still another advantage is securing a pre-scan lens of a pre-scan optics assembly against rotation about the optical axis, the process axis and the scan axis of a laser scanning device.